The present invention relates to an electrophotographic printing apparatus. More particularly, it relates to a multilayered photosensitive member of doped amorphous silicon formed on a printing drum. It further relates to a top or outer layer made of hydrogenated amorphous silicon carbide formed atop of the photosensitive member for protecting the member. It still further relates to the method for fabricating the top layer.
Electrophotography is a well-known technology, and electrophotographic printing apparatuses are widely used in the field. The apparatus has a photosensitive member disposed on the surface of a printing drum. In the first process step of the electrophotographic printing method, the photosensitive member is charged to a uniform potential to sensitize it using an electrostatic charger such as a corona discharger. The charged portion of the photosensitive surface is exposed to a light image of an original document which is to be reproduced. This records an electrostatic latent image on the photosensitive member corresponding to the original document. Thereafter, the latent image is developed by bringing a developing material such as a toner powder into contact therewith. In this way, a powder image is formed on the surface of the photosensitive member which is to be transferred onto a recording sheet. Thereafter, the powder image is permanently affixed to the recording sheet in the image configuration. Finally, at the next cleaning station, residual toner particles are removed from the surface of the photosensitive member. This is a typical cycle of the conventional electrophotographic printing process, taking approximately 20 seconds or so for a standard level apparatus, and several seconds for a high speed apparatus.
The photosensitive member of the printing drum described above consists of photosensitive, chargeable materials such as selenium or chalcogenide glasses (arsenic-selenium alloys and compounds). It is also known to utilize organic photosensitive materials therefor. Recently, however, amorphous silicon has become widely used, as for example, that disclosed in U.S. Pat. No. 4,507,375, issued on Mar. 26, 1985.
The requirements for devices of an electrophotographic printing apparatus are as follows. The material of the surface layer of the photosensitive member of the printing apparatus, which is formed on a printing drum, must have a high photosensitivity in the spectral range of the employed light source, such as a laser source. The material must also have a specific electrical impedance in darkness (dark resistance) of a magnitude greater than 10.sup.12 .OMEGA. cm, in order to substantially retain an electrostatic latent image thereon during at least one cycle of the printing operation, approximately 20 seconds as described above. The material must also have properties which remain unaltered with a continuous loading and unloading, i.e. which operates in a fatigue-proof manner and which is sufficiently resistant to abrasion during the printing operation, to various environmental hazards such as high humidity and to damages caused by the corona discharge. It is difficult to satisfy these requirements with a single photoconductive layer. For example, a photosensitive material having a high dark resistance and a high light conductance at the same time, is rarely found.
In order to fulfill these requirements described above, therefore, a multi-layered photosensitive member of amorphous silicon has been developed, being a well-known technology. One example is disclosed in U.S. Pat. No. 4,452,874 issued on June 5, 1984 to Ogawa et al. An amorphous layer II (top layer) of amorphous silicon carbide layer containing silicon atoms, carbon atoms, and hydrogen atoms, is disclosed therein, being accompanied with a method for forming the layer by electrically decomposing a gaseous mixture of hydrogenated silicon gases and hydrocarbon gases. A gas mixture of silane (SiH.sub.4) and methane (CH.sub.4) typically is used.
FIG. 1 is a schematic partial cross-sectional view of a multi-layered photosensitive member. On a cylindrical drum base 1 of conductive material such as aluminum, a charge blocking layer 2 of highly p-type doped hydrogenated amorphous silicon (a-Si:H) is formed by a conventional method such as a glow discharge CVD (chemical vapor deposition) method for decomposing and depositing gaseous mixture of silane (SiH.sub.4) and diborane (B.sub.2 H.sub.6) with electrical energy.
Over the charge blocking layer 2, a photoconductive layer 3 of slightly p-type doped hydrogenated amorphous silicon (a-Si:H) is formed by the same CVD method employing the similar gaseous mixture with a different gas ratio from the one of the preceding case. The photoconductive layer 3 has high electrical conductance under exposure to light (light conductance) but not so high dark resistance.
Thereafter, a top layer 4 is formed on the photocoductive layer 3 for not only protecting the surface thereof from various environmental hazards but also for retaining the charges of the electrostatic latent image formed thereon and for preventing the latent image from being dispersed and weakened. The top layer 4 is formed of a photosensitive material having a high dark resistance such as hydrogenated amorphous silicon oxide (a-SiO:H), hydrogenated amorphous silicon nitride (a-SiN:H), or hydrogenated amorphous silicon carbide (a-SiC:H). The top layer 4 has also high abrasion resistive properties sufficient to protect the surface from exterior mechanical damage during the operation.
The charge blocking layer 2 has a rectifying characteristics due to the difference of doping density between the charge blocking layer 2 and the photoconductive layer 3. Consequently, the injection of electrical carriers from the drum base 1 into the photosensitive member under dark condition is blocked and excess charges generated in the photoconductive layer 3 under exposure of light is allowed to flow from the photoconductive layer 3 to the drum base 1. Thus, by the aid of the charge blocking layer 2 and the top layer 4, the entire surface of photosensitive member has high dark resistance, being immune from any image flow or image weakening.
Among the above-described requirements for a photosensitive member formed on a printing drum, the following are characteristic of the top layer 4: a charge retaining capability for maintaining charges of a latent image recorded therein; resistance to deterioration when exposed to a corona discharge during the charging process, and resistance to abrasion and moisture caused by the exterior environment. However, these requirements have not been satisfied with a prior art top layer, causing some problems with the electrophotographic printing apparatus. The problems may be attributed to some defects in the top layer 4, such as small pin holes. Such defects of the top layer 4 are considered to be caused by some defective structure in the material of the layer 4.
Generally, the structural defects, namely, local distortion of the silicon network, of the amorphous silicon or amorphous silicon compounds such as amorphous silicon carbide, are caused by the presence of dangling bonds, that is, non-terminated bonds of silicon atoms. For example, in intrinsic amorphous silicon (a-Si), the distribution density of the dangling bonds is approximately 10.sup.20 cm.sup.-3. In hydrogenated amorphous silicon, three non-terminated bonds are intended to be bonded to hydrogen atoms (H). It is reported that the density of the dangling bond can be reduced to approximately 10.sup.15 cm.sup.-3 with an adequate fabricating method. However, the hydrogen atoms tend to be bonded to silicon or other atoms non-uniformly.
Particularly, in hydrogenated amorphous silicon compounds such as hydrogenated amorphous silicon carbide, hydrogen atoms are attracted and bonded to carbon atoms. The uniform distribution of bonded hydrogen atoms is desirable for reducing the structural defects in amorphous silicon compound material. Thus, a material having fewer dangling bonds therein and a method for fabricating the material are keys to improving the photosensitive member of the printing drum, and the solution of the above described problems.
Various efforts have been directed to solve the above problems regarding a top layer for an electrophotographic photosensitive member. The quality and the production efficiency depends on the combination and composition of the foregoing gas mixture. In a view to further improvement, namely, in order to achieve faster film formation rate and more resistance against the damage of the surface of the substrate (hereby, a photoconductive layer 3) caused by a glow discharge plasma during glow discharge CVD process, a new gaseous mixture and the resulting hydrogenated amorphous silicon carbide layer has been studied.
In the above description, a glow discharge CVD method is introduced for the formation of a photosensitive member. Other conventional methods, such as a sputtering method, and a laser assisted CVD method, are available for the same purpose. However, in the following, the glow discharge method will be described. The selection of the methods will depend on the quantity of production, variety of products, and investment for installed facilities.